Modern internal combustion engines generate approximately 20% of their hydrocarbon emissions by evaporative means, and as a result, automobile fuel vapor emissions to the atmosphere are tightly regulated. For the purpose of preventing fuel vapor from escaping to the atmosphere an Evaporative Emissions Control (EVAP) system is typically implemented to store and subsequently dispose of fuel vapor emissions. The EVAP system is designed to collect vapors produced inside an engine's fuel system and send them through an engine's intake manifold into its combustion chamber to get burned as part of the aggregate fuel-air charge. When pressure inside a vehicle's fuel tank reaches a predetermined level as a result of evaporation, the EVAP system transfers the vapors to a charcoal, or purge canister.
Subsequently, when engine operating conditions are conducive, a purge valve located between the intake manifold of the engine and the canister opens and vacuum from the intake manifold draws the vapor to the engine's combustion chamber. Thereafter, the purge canister is regenerated with newly formed fuel vapor, and the cycle continues.
As opposed to vacuum in naturally aspirated applications, at higher throttle levels a turbocharged/supercharged engine's intake manifold can see relatively high boost pressures generated by forced induction. Under this condition, a one-way check valve can be used to prevent backflow through the EVAP system and furthermore a vacuum ejector tee can be used to provide vacuum for purge flow.
In addition to a fuel vapor recovery function, an EVAP system may perform a leak-detection function. To that end, a known analog leak-detection scheme employs an evaporative system integrity monitor (ESIM) switch which stays on if the system is properly sealed, and toggles off when a system leak is detected. When the ESIM switch fails to toggle under specific conditions, an engine control unit (ECU) detects this situation and alerts an operator of the vehicle with a malfunction indicator.
Furthermore, an EVAP system's ability to detect leaks can be regularly verified in engine key-off mode via a so-called rationality test. Presently known rationality tests confirm the ESIM switch functionality through a simulated system leak which is generated by opening the purge valve to relieve a low level of system vacuum (approximately 0.5 KPa) retained from when the engine was running. The ECU then detects if the ESIM toggles from on to off, which is an indicator that the switch is functioning correctly. For the rationality test to be performed in a turbocharged/supercharged engine, however, a leak-detection scheme utilizing an ESIM switch has been heretofore known as requiring a two-way low airflow communication between the purge valve and the intake manifold. A simple check-valve does not permit two-way flow, therefore it will not support both purge flow during boost operation and ESIM functions in an EVAP system of a turbocharged/supercharged engine.